Development and Assessment of a Multi-Pipe Oil-Water Bulk Separator Concept for Subsea Applications
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Subsea produced water separation has emerged as a viable technology for tackling challenges arising from increased water production rates. Removing produced water at the seabed will free up topside capacity constrained water processing facilities, increase production rates, prolong field lifetimes and secure greener and more energy efficient oil and gas production. However, an important challenge connected to subsea produced water separation is that the cost of constructing, qualifying, transporting and installing subsea produced water separators often exceeds potential value gains in production. This is especially true for mature, marginal or deep-water fields. To make the business case of subsea produced water separation more attractive, there is a need for novel low-cost technologies suitable for standardization and modularization. In this PhD work, a novel concept for subsea oil-water bulk separation has been developed, a prototype of the developed concept has been constructed, and the concept has been evaluated both experimentally and numerically. The first phase of the research consists of a thorough state of the art review of available subsea produced water separator technologies. Drawbacks with existing technologies are outlined and focus areas for future technology developments are identified. Based on these findings, a new subsea produced water bulk separator concept, based on separation in multiple parallel pipes, is developed. For experimental testing, a down-scaled prototype of the separator concept has been constructed along with a low pressure two-phase oil-water test facility. The prototype consists of two 150.6 mm internal diameter pipes in parallel, with a total horizontal length of 6.1 m. Four experimental campaigns have been executed, focusing on performance and operational envelope mapping, design feature evaluation, flow distribution, control strategy development and effect of upstream inlet choking and addition of surfactants. Experimental fluids are Exxsol D60 and distilled water with added NaCl. Separator performance is determined by flow rate, density, temperature and pressure measurements, and pictures of flow phenomena and established inlet droplet distributions are gathered for supplementary analysis. Experimental results show that the prototype exhibits good performance for a wide range of inlet flow rates and water cuts. Based on results, design refinements are suggested and implemented, including preferred location for water extraction and an improved separator inlet configuration. An uneven flow splitting phenomenon is identified for certain flow conditions, which can be detrimental to operability and separator performance. Moreover, a robust control strategy to maintain satisfactory separator performance at varying inlet conditions is developed and implemented. Finally, quantification of performance variation due to more realistic inlet conditions are reported by studying the effect inlet choking and active interfacial agents has on separator performance. A computational fluid dynamics model of the prototype has been developed utilizing the commercial software Ansys CFX. The model assumes two-phase flow, where one phase is fully dispersed in the other as spherical droplets with uniform diameters. Phasic continuity and momentum equations are solved for each phase, included interfacial momentum transfer terms. Numerical model output displays fair agreement to experimental results for water-continuous regimes at the separator inlet. For oil-continuous inlet regimes, agreement was not satisfactory. The numerical model can be used for further refinement of the concept design. The presented research constitutes a scientific contribution to the oil and gas industry in the form of a developed, tested, and refined subsea oil-water bulk separator concept. This has been achieved by completing a thorough experimental and numerical study of oil-water separation and flow dynamics in parallel pipe geometries. The presented results indicate that the developed concept is attractive for further evaluation, and that it can form a basis for the development of next generation subsea produced water separators, overcoming outlined challenges with current technologies.